In the preparation of plywood and other board products made with veneer and phenol adhesives, the veneer is generally dried in an oven in order to lower the moisture content below 10%, otherwise when wet veneer, that is to say veneer with a moisture content greater than 11%, is hot pressed into plywood blows can occur in the panels due to steam formed by the moisture in the wood and consequently the adhesive does not bind the layers together. This results in the plywood panel not meeting the necessary strength requirements.
Because many sheets of veneer are placed in a single oven, they all receive the same drying time and temperature. However, it is known that sheets of green veneer when initially placed in a dryer do not have all the same moisture content. In fact there is quite a range of moisture content between veneer sheets and as a result the dried sheets have a range of moisture content, varying from bone dry veneer sheets to those having a moisture content in the order of 15%. The reasons for different moisture content of green veneer are many, and in one case include the fact that sapwood veneer has a higher moisture content than heartwood veneer.
If the veneer is over dried, that is to say if heating is continued after the moisture content is down to 0, then it is found that the wood veneer surface binds poorly with phenol formaldehyde resins.
When veneer sheets are dried below 10% moisture content, buckling or deformation can occur in the sheets, making the sheet hard to handle when being placed in a plywood press. The preferred range of moisture content for veneer sheets is between about 10 and 15%, but it is difficult to achieve this range in oven dried veneer. In most commercial veneer sheet production operations, the best range that can be economically achieved is plus or minus 5% moisture content and this results in a range of from 5% to 15% moisture content, although in most mills veneer sheets are dried down to 1% moisture content. As can be appreciated, drying veneer sheets to these low moisture content figures requires a lot of heating energy, and the dry sheets are often buckled or deformed which makes them more difficult to handle.